Mordekaiser's popularity is due to the rework
It's not that he's overpowered. It's that he as a champion is absolutely gnarly. He's thematically fulfilling, he's satisfying, everything about him feels good, from the autoattack noises to the recall animation. It's pure METAL and the entire Pentakill band was based around him, if you guys are surprised by a 90%+ presence in games after his rework then you don't know what hype is.